1. Field of the Invention
The present invention generally relates to an optical ring network, and in particular to a ring network having a protection capability using Wavelength-Division Multiplexing (WDM) technology.
2. Description of the Related Art
In an optical WDM ring network using a wavelength for services and another wavelength for protection, duplicate signals are generated by a node to travel over an optical fiber in opposite directions. In each node of the ring network, it is determined whether a signal can be received from another node on the wavelength for services. If a node receives the signal on the wavelength for services, the node selects the service wavelength to receive a transmission signal.
In case of a cable cut or a node failure occurring at a location, however, the node cannot receive the signal on the service wavelength from another node which is located between the failure location and the node in the downstream direction. In this case, the node selects the protection wavelength to receive the duplicate transmission signal traveling in the opposite direction. Therefore, data transmission can be performed as normal even if transmission failures occur. Such an optical WDM ring network has been disclosed in Japanese Patent Unexamined Publication No. 6-97950.
Other survivable WDM ring networks have been proposed by A. F. Elrefale (ICC'93 Geneva. 1993, Geneva, paper 48.7) and by R. E. Wagner et al. (OEC'94, 1994, Japan, 14C3-1).
However, the conventional ring network as described above has disadvantages in that the number of assigned wavelengths is twice that of nodes because a pair of wavelengths for service and protection are assigned to each node. This results in broadened necessary bandwidth. Further, a wavelength selector provided at each node has the increased number of wavelengths to be selected. Furthermore, since duplicate signals are traveling in opposite directions, a transmission line cannot be used effectively.